Arsonists set fire to a derelict farmhouse in Gorefield on Sunday.
Four crews of firefighters were called to tackle the blaze in Gore Dyke Bank.
Crews from Wisbech, Long Sutton with help from a turntable ladder from Dogsthorpe put out the blaze, believed to have been started between noon and 10pm.
Station Commander Jamie Johnson said: “The crews arrived to find a well developed fire with flames coming through the roof. They got to grips with it quickly to ensure the fire didn’t spread.
“The fire caused significant damage to the structure of the building. The roof and the first floor collapsed as a result, meaning we could not tackle the fire from the inside.
“The fire was started deliberately so if anyone has any information we urge them to contact the police on 101.”
Police are now appealing for more information. PC Matt Smart said: “It is clear this was a deliberate ignition as the electrics had been disconnected by the owner, therefore an accidental electrical fire has been ruled out by the fire service.
“Fortunately no one was hurt but under different circumstances it could have been fatal.” Anyone with information should contact PC Smart on 101 or Crimestoppers, anonymously, on 0800 555 111.